The present invention relates to an optical unit holding device for copying machines, more particularly, for the copying machines of the type of which optical unit is reciprocated for document exposure.
Most of the copying machines provided so far have been the type to expose the documents set on a stationary contact glass by moving back and forth an optical unit having a lamp and reflecting mirrors under the cotnact glass so as to enable compact design of the copying machine and high-speed copying.
Known as a type of reciprocate the optical unit in this manner is the optical unit holding device in which a slide shaft is passed through a hole formed on one end of the support frame to which the optical unit is attached, a slide member attached to the other end of the support frame is held on a slider, and a driving wire is wound around a pulley attached to the other end of the support frame to transmit driving force only to one side of the support frame.
With the optical unit holding device as described above, the support frame can be attached more accurately by making longer a length of the support frame through which the slide shaft is passed in an axial direction. Longer support frame where the slide shaft goes through, however, causes a problem since the support frame becomes longer and larger. If the length where the slide shaft passes through is made shorter to make the support frame smaller, on the other hand, parallelism of the optical unit may not be sufficient, causing a problem that the support frame is kept vibrated for a long time in the initial stage of moving for image forming, and the obtained copyed image is outstandingly deformed at the top thereof which corresponds to the initial stage of transfer in the optical unit.
Another optical unit holding device of known composition is of the type in which a slide member is attached to the bottoms of both ends of the support frame to which the optical unit is fixed, a pulley is attached respectively to each end of the support frame, the slide member at each end is held respectively on a slider, and a driving wire is wound around each pulley to transmit driving force to both sides of the support frame.
The optical unit holding device of the above composition can be such type, for example, in which three slide members are attached to the ends of the supprot frame so that all the slide members are put in contact exactly with the slider, or the type in which four slide members are attached to the ends of the support frame, and the support frame itself is made flexible so that all the slide members are put in contact with the slider (Japanese patent Publication (unexamined) No. 154434/1984).
All of the optical unit holding devices described above are advantageous in that the accuracy of the support frame itself need not be very accurate as the parallelism can be adjusted by winding the two driving wires and that the copying machine can be designed small-sized and lightweighted easily.
With the optical unit holding device of the 1st or former composition as described above, however, one end of the support frame to which driving power is transmitted by the driving wire is put in contact with and held onto the slider by two slide members, and the other end of the support frame is put in contact with and held onto the slider by one slide member only. When driving power is transmitted by the driving wires, therefore, one end of the support frame put in contact and held by one slide member only is greatly worn out, causing such problems as a generation of deflection of the supprot frame and the optical unit, degrading the quality of the obtained copied image.
With the optical unit holding device of the 2nd or latter composition described above, such deflection of the support frame can be avoided, because both ends of the support frame are respectively and exactly put in contact with and held onto the slider by the two slide members. If a weight balance of the support frame itself is disordered when the optical unit is attached to the support frame, however, inertia moment at transfer of the optical unit also differs as the support frame itself is flexible. Accordingly, the support frame itself is twisted by a disordered weight balance, causing deflection of the optical axis, so that a problem is generated wherein a quality of the copied image is degraded.
When the optical unit is reciprocated back from the exposure ending position to the home position, and again starts to move for exposure, a force is applied to the slide member in lifting direction upward from the slider at the exposure-start position adjacent to the home position. This also causes such troubles that the deflection at the top end takes place and image quality is degraded. When a speed of returning from the exposure end position to the home position is made faster to meet the demand for high-speed copying, in particular, the trouble mentioned above becomes more outstanding.
Moreover, a feeder wire is connected to the lamp to supply power. Within a specific range near the end position of forward motion of the optical unit determined by an attaching condition of the feeder wire, the radius of curvature of the feeder wire is smaller. This means that a force is applied to one of the pair of slide members which come in contact with the top surface of the slider in a direction to life up the slide member from the slider. Under that condition where the pair of slide members is forced upward, quality of the copied image is lowered due to deflection of the optical unit.
For the type of copying machines of which optical unit is moved, a power is transmitted from the driving source to the optical unit through wires for the advantages capable of simplifying a structure and easily obtaining a relationship between lamp transfer speed and reflector transfer speed at a ratio of 1:2. Some of them are so contrived to prevent exposure failure due to twist of the optical unit, or the like, by transmitting the power through wires to both sides of the optical unit. In this case, the optical unit is held onto the guide rail by its own weight to prevent deflection of the optical unit in vertical direction, and pulleys are respectively attached to both sides of the supprot frame supporting the optical unit and a driving wire is wound around both pulleys to apply downward force. To be more specific, the pulleys are vertically disposed and the wires are wound around the pulleys so that the pulleys can be rotated, and the pulleys are applied with downward forces through both wires wound around the pulleys, preventing lifting-up of the support frame (Japanese Patent Publication (unexamined) No. 37669/1983).
In the optical unit holding device of the above composition, pulleys are usually attached to the outside of the side plates enclosing the support frame. This arrangement, however, involves a problem that the range to accept electrical parts at outside of the side plates is narrowed or restricted by the range determiend by pulley diameter and optical unit transfer distance. Since diameter of the pulleys is substantially larger than the width thereof, the range determined by pulley diameter and optical unit transfer distance is rather extensive, which lowers the efficiency for using the space within the copying machine.
Another optical unit holding device known is the type in which both pulleys are allowed to rotate freely within the horizontal plane, a wire is wound around the pulleys in the same direction between the two pulleys and the center of gravity of the support frame is set at the same height as the center of gravity of the pulleys so as to prevent lifting up of the support frame (Japanese Patent Publication (unexamined) No. 156965/1984).
With the optical unit holding device composed as described above, however, the center of gravity of the support frame comes between the pitch of wire winding, since the wires are wound around the pulleys by one turn only, although the center of gravity is set at the same height as that of the pulleys. Accordingly, the position where driving power of the wire applies does not coincide with the gravity position of the support frame, and since the wires are wound in the same direction around both pulleys, a deflected tensile force is applied to the support frame. This causes a problem of lifting the support frame from the guide rail.